depends on what you want, there are a few ways.
First, you can change the part of the unit that changed color depending on the Player(red, blue, green)
1) Open Object Editor
2) Click the unit you want to change
3) Scroll down to "Art-Team Color" And change it.
note- Hostile is black i think
The other way is to change the color totally, its kinda strange but good for making units black.
Steps, follow 1 and 2 from befor
And scroll down to "Art-Tinting Color (Red)&(GReen)(Blue)". The Defult is 255, just the lower you have of one the lower the tint of that color. 0,0,0 is black xD
The other way is to skin, there are lots of tutorials on that.
Hope that helps